The present invention relates to a voltage regulator for a power supply device, and more specifically to an improved low noise voltage regulator using digitally controlled switching techniques.
Switching voltage regulators are commonly used in power supply devices, such as those used in consumer appliances, e.g., personal computers, electronic appliances, etc. FIG. 1 generally illustrates the form of a current evolution of a three-phase voltage regulator shown at reference numeral 10 designed for better line and load regulation.
A three-phase voltage regulator 10 comprises a single pulse width modulation (PWM) control integrated circuit (IC) 20 and three channels 30(1), 30(2), and 30(3). Each channel 30(i) comprises a driver circuit 40(i), a high-side supply transistor 42(i), a low-side sink transistor 44(i) and an energy-storage inductor 46(i). In operation, the PWM control IC turns on each individual transistor in the channels on a synchronized schedule to manage the timing of the storage of energy in the resonant circuit formed by the corresponding inductor in that channel and a common capacitor 48 shared by all of the channels.
Exemplary waveforms for the three-phase switching voltage regulator system 10 are shown in FIG. 2 and illustrate the concept of interleaved switching to control the voltage regulator output. The width of each PWMn pulse (PWM1, PWM2, PWM3) controls the duration of the conduction period for the respective high-side supply transistors 42(i) in the corresponding channel 30(i). When each PWMn pulse is low, the respective low-side sink transistor 44(i) conducts in the corresponding channel 30(i). The driving signal to the low-side sink transistor 44(i) may optionally be derived separately from the driving signal for the high-side source transistor 42(i) so that its timing may be independently adjusted. Using well-known pulse width modulation techniques, the width of each PWMn pulse is adjusted to control the amount of energy stored in the inductor for the associated channel. This in turn controls the transfer of that energy to the common capacitor and, consequently the output voltage of the regulator. The positive series reactance of the separate channel inductors and the negative reactance of the shared capacitor also provide a filtering action that removes switching artifacts from the regulator output, providing a relatively steady, direct-current (DC) voltage.
The purpose of the low-side transistor 44(i) is to supply current to the corresponding inductor 46(i) from the circuit ground, when the current supplied by the high-side source transistor 44(i) is off. If the low-side transistor current path was not provided, the voltage on the “near side” of the inductor 46(i) would rise until it broke down a path to a current source. This is a result of the fact that the current through an inductor must be continuous, but the voltage across it may change instantaneously.
The low-side transistor needs to be turned off sufficiently in advance of the high-side transistor being turned off to avoid the voltage breakdown problem. This results in “shoot-through” current from the regulator input voltage to ground through the two transistors, which reduces the regulator's overall efficiency.
Several advantages can be realized by increasing the frequency of the driving signal pulses and the number of phases in the voltage regulator. Since the resonant frequency of the separate channel inductors and the common capacitor is given by,
      f    PWM    =      1          2      ⁢      π      ⁢                                    L            channel                    ⁢                      C            common                              Increasing the pulse frequency allows reduction of the values of the inductors and capacitor to provide equivalent filtering of switching artifacts in the regulator's output. Use of smaller inductors and capacitors eases physical placement constraints and reduces the total circuit area consumed by inductors and capacitors.
With more phases, less current is required of each channel because the total current is shared across more channels, reducing energy loss and heat generation resulting from both conductor losses in the inductor wiring and flux losses in the inductor's magnetic core material. Decreasing the current switched at any instant in time also reduces the amount of electromagnetic interference (EMI) that the circuit generates and allows the switching transistors to be air-cooled while mounted in a vertical position to further save circuit board area.
Operating more phases necessitates reduction of the maximum duty cycle of each individual phase to prevent the time overlap of signals in any two phases which would interrupt the proper scheduling of energy delivery from the separate inductors in each channel to the common capacitor. According to Fourier theory, reducing the duty cycle of a pulse train increases the range of frequencies over which the spectral energy produced by the pulse train is spread. This reduces the amount of spurious energy produced at any single frequency.
Nevertheless, increasing the pulse frequency and the number of phases places more stringent constraints on the timing of individual PWMn pulses. A common two-phase system operating at 200 KHz requires a PWMn pulse to be generated every 2.5 μs. Controlling pulse width to within 1% to provide the necessary load regulation requires timing control of 25 ns. Likewise, a sixteen-phase system operating at 10 MHz to realize the benefits described above requires timing control of 62.5 ps, which is well beyond the capabilities of today's digital multiphase switching voltage regulator systems. Even more precise control of separate driving signals to the high-side source transistor and the low-side sink transistor must be maintained to prevent voltage breakdown of the transistors while reducing efficiency losses due to shoot-through current from the regulator input to ground through the two transistors. Moreover, the repeated generation of the energetic PWMn pulses produces EMI at both the fundamental frequency of the pulse generation, the inverse of the repetition rate, and at harmonics of this frequency.
There is room for significantly improving voltage regulators and more particularly to reducing EMI and enhance performance of a switching voltage regulator. By monitoring the state of the voltages across the high-side and low-side transistors and taking advantage of precise timing capability, it is possible to increase the efficiency of the voltage regulator without exposure to voltage breakdown.